YbCu
4Ag crystallizes in the cubic AuBe
5-type structure where Yb is trivalent and shows dense Kondo behavior, while YbCu
5 is known to have the hexagonal CaCu
5-type structure where Yb is in a nonmagnetic divalent state. In this study, a pseudo-binary system YbCu
5-xAg
x was investigated. The powder Xray diffraction revealed that YbCu
5-xAg
x has the cubic AuBe
5-type structure in the range of 0.125≤x≤1.0, while both the cubic and the hexagonal phase exist for 0≤x≤0.1, which suggests YbCu
5 has also cubic AuBe
5-type where Yb seems to be trivalent. Magnetic and transport properties show that YbCu
5-xAg
x is a dense Kondo system in which Kondo-temperature T
K decreases continuously with the decrease of Ag-concentration x, causing the enhancement of the zerotemperature susceptibility x(O), the T
2-coefficient A of electrical resistivity and the Sommerfeld coefficient γ of specific heat. The γ value was found to be enhanced to as large as 460mJ/mo1K
2 in YbCu
4.875Ag
0.125. These results imply the cubic AuBe
5-type YbCu
5 is a dense-Kondo or a heavy-fermion system, in which γ reaches to about 500mJ/molK
2.
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